The present invention relates generally to the ability to improve the surface smoothness on composite materials for use in the construction industry. More particularly, the present invention relates to the use of air jets to impart energy to the surface of the material when the composite material is still in a slurry or semi-slurry state where such energy acts to break up flocs or clumps of composite material that otherwise would render a rougher surface on the finished product.
The United States Gypsum Company""s gypsum fiberboard process, as shown and described in U.S. Pat. No. 5,320,677, and herein incorporated by reference in its entirety, describes a composite product and a process for producing a composite material in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum, i.e. calcium sulfate in the stable dihydrate state (CaSO4.2H2O), to calcium sulfate alpha hemihydrate having acicular crystals. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the mat is pressed into a board of the desired configuration. The pressed mat is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried.
One of the many advantages of the process disclosed in the ""677 patent is that the surface of the resulting gypsum panel can be smoothed, or in the alternative, textured as the panel is being formed. The challenge in treating the surface of gypsum fiberboard during in-line processing is the timing of the treatment made on the slurry or wet mat. The smoothing as taught in this application begins while the material is still in a slurry or just beginning to form a semi-slurry state.
As the rehydratable calcium sulfate hemihydrate and cellulosic fibers in a slurry form leave the head box, and are disposed upon the conveyor belt or forming wire, the slurry will have a temperature generally in the range of about 200xc2x0 F.+/xe2x88x9210xc2x0. Thereafter, as the slurry is spread to create a forming pond across the conveyor the action of vacuum pumps begins removal of the free water and the temperature drops significantly and the rehydration process begins.
As the slurry exits the head box, the dewatering process begins with the action of vacuum pumps. However, commingled crystals and fibers may collect and form clumps or flocs, which is undesirable at the surface of the product. It is preferred that the clumps or flocs have a greatest dimension less than approximately 6 mm. When rehydrated, clumps of commingled crystals and fibers larger than 6 mm impart an undesirable roughness to the surface of the finished material. Roughness of wet felted products is detrimental to final installations where surface finish is important to final applications, such as painted surfaces (walls) and thin overlays (vinyl laminations). One contributor to such roughness is the condition present in the forming pond during substrate manufacture.
Typically at least two factors increase roughness: high consistencies and long fiber content. These are known to be minimized by the addition of water to lower consistencies or by agitation in the slurry pond. Both of these methods though have other undesirable effects. The addition of water adversely affects drainage rates and may cause line speed decreases and increase vacuum demands. Agitation in the pond may adversely affect the preferred form of matrix formation which is collective sedimentation as the slurry is dewatered into a filter cake if the agitation is applied in the wrong stage of formation of the product or at the wrong level in the pond. In addition, if the agitation is used with a slurry having a raw material mix of widely divergent densities or settling rates, such as is common with high filler fine paper or wet process gypsum fiberboard, the lower density material will separate from the higher density material, causing a nonuniform product in the case of the wet process gypsum fiberboard. The separation of materials may also result in a decreasing first pass retention on the forming wire as higher density materials that should remain on the forming table are drawn off by the vacuum boxes during dewatering and returned upstream of the head box for recirculation into the slurry.
A variety of other methods have been attempted to modify forming pond characteristics to improve surface smoothness, such as vibrating rods, vibrating plates, rotating rolls, smooth top plates, water sprays, etc. The use of an air lance directed upstream, but located downstream of the forming pond has yielded the best smoothness with the unexpected result of improved flexural strength.
The present invention relates generally to producing gypsum fiberboard panels with a smooth surface texture. More particularly, the present invention relates to the use of jets of air to impart a smooth surface texture to gypsum fiberboard panels.
An air knife or air lance with proper pressure, angle of incidence and distance from surface, may be applied in the formation pond of wet felted products, particularly those formed at high consistency, to modify slurry properties during formation and improve properties, in particular surface smoothness of the final panel.
With reference to the ""677 patent, energy is selectively applied to the top of the forming pond close to the wet line by an air lance of sufficient energy to disperse clumps in the pond surface and slightly below it, but insufficient energy to disrupt the pond more than slightly below the surface. This energy disperses clumps or flocs during mat formation without disrupting the preferred method of collective sedimentation.
The air lance extends across the forming pond from one side dam to the opposing side dam. The air lance is applied at a low incident angle and so gives much greater flexibility than a water spray, although a water spray is an effective alternative. In addition, the air lance has the advantage over the water spray of no water addition and subsequent required removal by the dewatering process.
One method of generating this air lance is via WINDJET brand nozzles manufactured by Spraying Systems Inc. These nozzles are installed in a manifold to yield a uniform airflow along a two dimensional path. Experimentation with various pressures and flow levels indicated better performance at lower pressure/flow levels. The lower pressures required for optimal results allows the air lance to utilize a blower as a source of air flow rather than more expensive air compressors. Experimentation has also shown an unexpected result of a higher bending strength value in the resulting gypsum fiberboard product.